1. Field of Invention
The present invention relates generally to production and reproduction of images, and more particularly, to enhancing the quality of images produced by using a tone reproduction curve which has been selected on the basis of psychophysical data.
2. State of the Art
In the production and reproduction of imagery using devices such as video displays and printing devices, image analysis techniques are typically relied upon to improve quality. For example, image acceptability is often evaluated with respect to a reference image in a three dimensional color space, wherein a location of each color is defined by three dimensions. One such color space is the internationally standardized CIELAB LCh color space, wherein each color location is defined by its lightness, chroma and hue angles. Image evaluation in a given color space can be used to provide a tone reproduction curve that is embedded in an image production device to enhance the quality of resultant images.
Tone reproduction curves constitute transfer functions for modifying one or more dimensions of an image which is to be produced. For example, linear or nonlinear transfer functions can be used to modify the lightness of an image.
One transfer function used as a tone reproduction curve is the multiplicative factor transfer function. With the multiplicative factor transfer function, the lightness of an input image is multiplied by a constant to provide an output image with a modified lightness. A tone reproduction table can then be created using the multiplicative factor, and stored as a look-up table in an image production device.
The tone reproduction curve modifies lightness of the output image to compensate for distortion which results from, for example, characteristics of the display or printer, a printing substrate, printing ink characteristics and so forth. If left uncorrected, the unmodified lightness could result in output image which appears too dark, too shadowed or which lacks adequate contrast.
Tone reproduction curves for correcting lightness have achieved widespread use in, for example, gray scale printers. However, there is currently no efficient and highly accurate technique available for evaluating plural tone reproduction curves in an effort to select the one curve which will produce output images with the most visually pleasing results. Rather, a tone reproduction curve is typically selected by comparing test images (which have been produced using the curve) to known reference images. Discrepancies are then evaluated on the basis of half toning concerns such as dot gain, or on the basis of predetermined measurement constraints, such as an ink density (for example, an effort can be made to linearize the dot gain or density of a gray scale which has been printed). Other techniques for selecting a tone reproduction curve rely on uncontrolled, ad hoc assessment of test images to identify a tone reproduction curve which appears to provide a visually pleasing result.
Thus, techniques for selecting a tone reproduction curve require some form of matching to a predetermined constraint, (such as linearizing a printed gray scale), or involve uncontrolled, ad hoc assessment of test images. However, these techniques fail to assure that the subjectively best tone reproduction curve will be selected. Further, to provide at least a minimum level of accuracy, these techniques require using a highly skilled engineer to select the tone reproduction curve and develop an associated tone reproduction table.
Accordingly, the present invention is directed to producing or reproducing an image using a tone reproduction curve which has been selected on the basis of a statistical evaluation of psychophysical data, such as psychophysically quantified, subjective judgements. By providing a controlled acquisition and evaluation of psychophysical data to select a tone reproduction curve, improved imagery can be assured. Further, once a procedure for controlled acquisition and evaluation of psychophysical data has been established, lesser skilled technicians can be used to carry out selection of a tone reproduction curve.
Exemplary embodiments are directed to a method for producing an image comprising the steps of: selecting at least one tone reproduction curve by statistically analyzing psychophysical data acquired using a plurality of tone reproduction curves, and producing the image using the at least one tone reproduction curve.
Further, exemplary embodiments are directed to an apparatus for producing an image comprising means for storing at least one tone reproduction curve having a parameter based on statistical analysis of psychophysical data acquired using plural tone reproduction curves, and means for producing an image using the at least one tone reproduction curve.